Can anyone explain what are thumbnails? What do they do in a computer programme?
"Thumbnail" means something small, the size of a thumbnail. It essentially is simply an icon that not only shows you what kind of a file it it is, but also shows you a small image of the file.
So for example if you have thumbnail of a jpg file, it not only shows you that it's a jpg file, it also shows you a miniature picture of the image the file contains.